This invention relates to superconducting circuits employing devices such as those known as Josephson junctions (JJs).
It is known in the art to employ JJs integrated together according to a rapid-single flux-quantum (RFSQ) methodology to manufacture ultrafast superconducting digital circuits.
The RSFQ technology is a low-voltage technology producing pulses having amplitudes in the range of 0.5 millivolts (mV) which are very fast (e.g., 4 picoseconds wide). In order to transmit the digital information generated by and within these RSFQ circuits for processing by standard semiconductor circuits, several stages of amplification are needed to increase the amplitude of the pulses while maintaining high speed of operation without introducing noise and distortion.
A prior art superconducting circuit for amplifying signals is shown in FIG. 1. FIG. 1 is a highly simplified schematic diagram which shows a current source, 10, supplying a current (Ics), to a stack 12 of superconducting quantum interference devices (SQUIDs) connected in series between an output terminal 17 and a point of reference potential 19, shown as ground. A load resistor RL is connected in parallel with the stack 12. FIG. 2 shows the stack 12 includes “n”, generally identical, SQUIDs (12a through 12n) connected in series with the same SQUID current (Ics) flowing through each SQUID of the stack 12. Each SQUID (see for example 12i) includes an input node, 121, and an output node 123 and two JJs (JJ1, JJ2) interconnected (in parallel) by inductive elements defining an inductive loop. SQUIDs have a settable critical current (Ic) and are characterized (as shown in FIG. 3) such that: (a) when the SQUID current (Ics) flowing between the input and output nodes of a SQUID is below the critical current (Ic) of the Josephson Junctions of the SQUID, the SQUID is in its superconducting state, exhibiting essentially zero resistance between its input and output nodes; and (b) when the current, Ics, flowing between the input and output nodes of a SQUID is above the critical current (Ic) of the SQUID, the SQUID is placed in its resistive state, exhibiting a finite resistance (e.g., 1 ohm) between its input and output nodes. A control current (Icc or Is) may be electromagnetically coupled to a SQUID, as shown in FIGS. 1 and 2, such that the SQUID's critical current (Ic) is raised to a higher value (Ic=Ich) or is decreased (i.e., depressed) to a lower value (Ic=Icd). Thus, for a given Ics flowing through a SQUID, it can be driven between a superconducting state and a resistive state by varying the Icc electromagnetically coupled to the SQUID.
FIGS. 1 and 2 show a critical current control circuit (a signal generator) 14 whose current output, designated as Is or Icc, is distributed via a line 15 to the SQUIDs of the stack for electromagnetically controlling/setting the value of the critical current (Ic) in the SQUIDs of the stack.
For purpose of illustration it may be assumed that the resistance of a single SQUID, in its resistive state, is in the range of one (1) ohm. In order to amplify the signal and generate signal voltages of several millivolts it is necessary to have many SQUIDs stacked in series. Assume, for purpose of example, that 50 SQUIDs are stacked in series and, when in the resistive state, the SQUID stack has a total resistance of Rs and that an RL of 50 ohms is connected in parallel with the stack.
When the stack 12 is in its superconductive state, the output line 17 is clamped to ground. When the stack 12 is in the resistive state, the voltage on output line 17 is equal to [Ics]×[(Rs)(RL)/(Rs)+(RL)]. This enables the production of a unipolar amplified signal. However, the prior art circuit suffers from some significant limitations in that the output for the resistive state condition is not well defined. That is, the output signal condition is a function of the value of, and limitations, on Ics and of the Rcs of the stack. For a given Ics, within the range of Icd and Ich, flowing through a SQUID, the SQUID can be driven between a resistive state and a superconductive state. However, if Ics is made greater than some value of Ich, the SQUID can not be readily switched from the resistive state to superconductive state. Thus, the prior art circuit provides for a degree of signal amplification, but does not ensure that the output signal is driven to a known and fixed voltage level when the SQUIDs in the stack are in the resistive state.
When the SQUIDs in a stack are in the superconductive state, they have essentially zero impedance. Therefore, if the SQUIDs in a stack were driven by a voltage source excessive currents would flow through the short circuit (zero impedance) condition. Thus, in accordance with the prior art, as shown in FIGS. 1 and 2, the SQUIDs in a stack are driven by a current source whose current, Ics, flows to ground either via a short circuit or via the parallel combination of the stack resistance in parallel with the load resistance. The stack of SQUIDS cannot be driven by a voltage source because of the short circuit condition which would cause undesirably large currents to flow.
Another problem with the prior art circuit (see FIG. 2) is that it is desirable for all the SQUIDs in the stack to be turned on to one condition, or another, at the same time. However, when there are a large number of SQUIDs, it is difficult to distribute the control signal to achieve this result. For example, in FIG. 2 the control line 15 is shown to be wound around all the SQUIDs of the stack so as to distribute the control signal in a serial fashion to the SQUIDs. At the operating frequencies of interest even small differences in the length of the control signal line between different ones of the SQUIDs results in different propagation delays and the application of set or reset signals at different times to the different SQUIDs. The operation of the stack and the speed of response are then adversely affected.
Accordingly, it is desirable to have an improved amplifier which can provide ultra fast voltage amplification reliably.